• Porphyridium cruentum;
  • photosynthetic efficiency;
  • quantum demand;
  • light-limited culture;
  • maintenance energy


Steady state growth of the red alga Porphyridium cruentum was obtained in light-limited chemostat cultures at different temperatures, pH and salinities over a range of specific growth rates. Geometric design of the reactors was found to have no effect on the yield and photosynthetic efficiency (PE) of the biomass. Under optimal growth conditions, that is at 23 °C, pH 8 and NaCl concentration of 0.42 M, the maximum specific growth rate was 0.065h−l which corresponded to a generation time of 10.7 h. With a given light input the biomass output rate was found to be constant over the range of specific growth rates studied. The values of the maximum growth yield (YG) and maximum PE of P. Cruentum were in the range of 0.0064–0.0074 g kj−1 and 13.4–15.5% respectively, when growth was optimal. This PE value corresponds to a quantum demand of 15 per O2 molecule evolved. Different temperatures, pH and salinities were demonstrated to cause a 1.5 to 2-fold variation in the maximum PE. The maintenance energy was found to be virtually zero under all conditions.